Haptic controller for road vehicles

ABSTRACT

An electric assisted steering system for a motor driven road vehicle, having assist torque signal generating means which generates an assist torque signal for the steering system in response to the driver&#39;s applied torque and sensed vehicle speed to reduce the driver&#39;s steering effort. A yaw rate haptic torque is generated which is based upon vehicle rate error and is arranged to be added to the torque assist signal such that, when the yaw rate error builds up corresponding to increasing steering instability (e.g. understeer or oversteer) of the vehicle, the haptic torque added to the torque assist signal reduces the effective road reaction feedback sensed by the driver in advance of any actual vehicle stability loss whereby to allow the driver to correct appropriately in good time before terminal steering instability is reached. Alternatively, the haptic torque can be based upon vehicle lateral acceleration which is arranged to be subtracted from the torque assist signal such that when vehicle lateral acceleration builds up, corresponding to tighter cornering of the vehicle, the haptic torque subtracted from the torque assist signal increases the effective road reaction feedback sensed by the driver corresponding to the increase in cornering forces generated by the tyres of the vehicle.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/GB02/00523 filed Feb. 7, 2002, the disclosures of which areincorporated herein by reference, which claimed priority to GreatBritain Patent Application No. 0103015.4 filed Feb. 7, 2001, thedisclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to electric assisted steering systems(EAS) in motor driven road vehicles and is concerned in particular witha control system in a road vehicle adapted to provide steering torquecompensation or haptic torque based on the measured vehicle dynamics,such as yaw rate or lateral acceleration.

Electric assist steering systems are well known in the art. Electricassist steering systems that use, for example, a rack and pinion gearset to couple the steering column to the steered axle, provide powerassist by using an electric motor to either apply rotary force to asteering shaft connected to a pinion gear, or apply linear force to asteering member having the rack teeth thereon. The electric motor insuch systems is typically controlled in response to (a) a driver'sapplied torque to the vehicle steering wheel, and (b) sensed vehiclespeed.

Other known electric assist steering systems include electro-hydraulicsystems in which the power assist is provided by hydraulic means underat least partial control of an electrical or electronic control system.

There is a desire, at least in certain vehicle market segments, toprovide the driver with information about the dynamic state of thevehicle via the steering wheel torque. The effects that are most commonare an increase in driver torque as the lateral acceleration on thevehicle increases (the handwheel seems to become heavier), and a suddendrop in driver torque (the handwheel seems to become much lighter) whenthe vehicle reaches terminal understeer. (Terminal understeer isconsidered to be when an increase in handwheel angle, no longer gives anincrease in vehicle yaw rate.) Traditionally these effects have beenproduced by careful design of the steering system, but modern powerassisted steering systems, and space and other compromises in the designof steering systems, has lead to the effects becoming much lessnoticeable. However, there is a general perception that these effectsimprove the handling of a vehicle, and therefore they can be quiteimportant in certain market segments.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the hapticinformation that the driver receives from the steering system and toprovide an algorithm using information already contained in the carabout the dynamic state of the car, to artificially recreate thesteering feel properties described above.

In its broadest scope, the invention is concerned with a controlalgorithm that uses signals from motion sensors to adjust the assisttorque provided by a power assisted steering system in such a way thatinformation about the dynamic state can be gleaned from the feel of thesteering. In one embodiment, the algorithm recreates the increase indriver(handwheel) torque felt by a driver in proportion to lateralacceleration, and in another embodiment the decrease in driver torquewhen the vehicle enters terminal understeer.

In accordance with a first aspect of the present invention, there isprovided a power assisted steering system for a motor driven roadvehicle, the system including assist torque signal generating meansarranged to generate an assist torque signal for the steering system inresponse to the driver's applied torque and sensed vehicle speed andeffective to reduce the driver's steering effort and a means forgenerating a haptic torque based upon vehicle yaw rate error which isarranged to be added to the torque assist signal such that when the yawrate error builds up, corresponding to increasing steering instabilityof the vehicle, the haptic torque added to the torque assist signalreduces the effective road reaction feedback sensed by the driver inadvance of any actual vehicle stability loss whereby to allow the driverto correct appropriately in good time before terminal steeringinstability is reached.

Such a system has the advantage of drawing steering instabilityconditions (e.g. understeer or oversteer) to the attention of thedriver.

Preferably, the assist torque signal generating means comprises anelectric motor.

Yaw rate error can be established by comparing an estimated yaw ratederived from measured values of steering angle and vehicle longitudinalvelocity, with measured vehicle yaw rate.

Preferably, the yaw rate error is saturated, if necessary, to preventexcessive demand and scaled by a gain map.

The gain is preferably controlled in accordance with yaw rate error,such that a low yaw rate error results in a relatively low gain and ahigh yaw rate error results in a relatively large gain so as to increasethe assist torque from the power steering and make the steering feellight to the driver.

In some embodiments, a plurality of gain maps are provided, the mostsuitable to comply with the prevailing conditions being arranged to beselected automatically from a judgement of road surface conditions basedon measured data, such as measured yaw rate error and column torque.

The haptic torque is preferably established by scaling the steeringcolumn torque using the scaled yaw rate error, the haptic torque beingadded to the torque assist to provide an output for driving the electricmotor.

A dynamic yaw rate error signal can be derived from a dynamic yaw rateestimation; a functionally equivalent lateral acceleration error signalcan also be derived from an equivalent dynamic lateral accelerationestimator.

Thus, in accordance with a second aspect of the present invention, thereis provided a power assisted steering system for a motor driven vehicle,the system including assist torque signal generating means arranged togenerate an assist torque signal for the steering system in response tothe driver's applied torque and sensed vehicle speed and effective todecrease the driver's steering effort, and a means for generating ahaptic torque based on vehicle lateral acceleration which is arranged tobe subtracted from the torque assist signal such that when vehiclelateral acceleration builds up, corresponding to tighter cornering ofthe vehicle, the haptic torque subtracted from the torque assist signalincreases the effective road reaction feedback sensed by the drivercorresponding to the increase in cornering forces generated by the tyresof the vehicle.

This latter arrangement enables the controller to be further tuned togive a heavier steering feel up to the point of understeer followed by alowering in column torque once impending understeer has been determined.

In accordance with a third aspect of the present invention there isprovided a power assisted steering system for a motor driven roadvehicle, the system including assist torque signal generating meansarranged to generate an assist torque signal for the steering system inresponse to the driver's applied torque and sensed vehicle speed andeffective to reduce the driver's steering effort, and a means forgenerating a haptic torque based upon vehicle lateral acceleration errorwhich is arranged to be added to the torque assist signal such that whenthe lateral acceleration error builds up, corresponding to increasingsteering instability of the vehicle, the haptic torque added to thetorque assist signal reduces the effective road reaction feedback sensedby the driver in advance of any actual vehicle stability loss whereby toallow the driver to correct appropriately in good time before terminalsteering instability is reached.

In accordance with a fourth aspect of the present invention, there isprovided a power assisted steering system for a motor driven roadvehicle, the system including assist torque signal generating meansarranged to generate an assist torque signal for the steering system inresponse to the driver's applied torque and sensed vehicle speed andeffective to reduce the driver's steering effort, and a means forgenerating a haptic torque based either upon (a) vehicle yaw rate erroror (b) lateral acceleration error which is arranged to be added to thetorque assist signal such that when said error builds up, correspondingto increasing steering instability of the vehicle, the haptic torqueadded to the torque assist signal reduces the effective road reactionfeedback sensed by the driver in advance of any actual vehicle stabilityloss whereby to allow the driver to correct appropriately in good timebefore terminal steering instability is reached.

In the latter case, therefore, the overall control method is the samefor providing the haptic torque but that the pre-processing part of themethod optionally takes either yaw rate or lateral acceleration as thecontrolling input and where the use of lateral acceleration additionallyprovides further benefits in providing a haptic torque up to the pointof impending understeer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating one embodiment of a controlsystem in accordance with the present invention;

FIG. 2 shows examples of controller gain maps that can be used in thepresent invention;

FIG. 3 illustrates a derivative function of the steady state controllerof FIG. 1;

FIG. 4 substitutes lateral acceleration for yaw rate;

FIG. 5 illustrates the provision of a modifying torque based on lateralacceleration;

FIG. 6 is a block diagram illustrating a further embodiment of a controlsystem in accordance with the present invention;

FIG. 7 is a block diagram illustrating a further embodiment of a controlsystem in accordance with the present invention;

FIG. 8 illustrates yaw rate estimation and reverse detection;

FIG. 9 is a block diagram of a yaw rate estimator;

FIG. 10 is a block diagram of a lateral acceleration estimator;

FIG. 11 is a block diagram illustrating reverse detection;

FIG. 12 is a block diagram illustrating understeer control;

FIG. 13 illustrates a yaw rate error to driver torque scaling look-uptable;

FIG. 14 is a block diagram of a torque control section;

FIG. 15 illustrates scaling correction shape;

FIG. 16 illustrates lateral acceleration feel control; and

FIG. 17 is a block diagram of a further embodiment similar to FIG. 7 butwith the yaw rate estimator replaced by a lateral accelerationestimator.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, the vehicle steer angle and longitudinalvelocity are input to element 10 where an estimate is established of theyaw rate demanded by the driver of the vehicle. The yaw rate estimationis based for example on the steady state understeer equation, expressedas:

$\hat{r} = {\frac{V_{x}}{l\mspace{11mu}\left( {1 + \left( {V_{x}/V_{c\; h}} \right)^{2}} \right)}\frac{\delta_{sw}}{G_{s}}}$where V_(X) is the vehicle longitudinal velocity, l is the wheel base,V_(ch) is the vehicle characteristic speed, δ_(SW) is the handwheelangle and G_(S) is the gain of the steering system from road wheels tohandwheel. This estimated value is then passed through a first order lowpass filter, tuned to give the estimate similar lag to the vehicle.Careful selection of the break point in this filter allows the pointwhere the steering goes light in relation to the loss of steer authorityto be controlled.

The resulting estimated yaw rate is compared at 12 with a signalrepresentative of the actual vehicle yaw rate, as measured by a VehicleStability Controller (VSC) or similar sensor, to generate a yaw rateerror signal on line 14. The yaw rate error is then saturated at 16 toprevent excessive demand and scaled by a gain map 18.

The saturation block 16 prevents the yaw rate error from reaching toohigh a level. Experience has shown that if the yaw error is allowed toincrease too much, then this can excite an instability in the EASsystem. This may be dependent to some extent on the particularcharacteristics of the EAS system fitted to the vehicle, but thesaturation also prevents the system producing excessive torques in theevent of an error. It may be necessary sometimes to tune the value ofthis saturation in dependence upon the surface that the vehicle is on. Apossibility is to use column torque and yaw rate error as indices into alook-up table.

The gain at 18 is varied in accordance with the yaw rate error. A lowyaw rate error indicates the linear regime referred to above in whichthe vehicle is operating at constant forward speed and before terminalundersteer is reached, and where therefore a low gain is required. Onthe other hand, a high yaw rate error is indicative of excessiveundersteer, and therefore a large gain is required to increase theassist torque from the power steering and make the steering feel light.

The haptic torque on line 20 is established by using the scaled yaw rateerror signal from the gain map 18 to scale at 28, the column torque(Tcol) which has been low pass filtered at 22 to prevent excitingunstable modes in the power steering. By scaling the column torque at28, rather than adding to it, the controller is prevented from enteringa region where it may attempt to drive the steering system against thedriver. In this manner, the inherent self-centring of the steering ismaintained. If the driver releases the steering wheel, then the columntorque falls to zero, and the haptic torque also falls to zero.

The output from the controller is thus determined by multiplying at 28the column torque (Tcol) with the output from the gain map 18. Asdescribed above, the column torque is low pass filtered at 22, again toprevent excitation of the EAS unstable modes. Careful design of thisfilter 22 may show that is possible to guarantee stability and allow theremoval of the saturation element 16. The result of the multiplicationat 28 is then added to the assist torque (Tassist) generated from thepower steering controller, and fed to the power steering (EAS) motor.

The “Abs” blocks 24 and 26 in FIG. 1 are included so that the absolutevalues, or magnitudes, of the input signals are taken.

The yaw rate error in the arrangement of FIG. 1 is scaled by a gain mapat 18 in establishing the final output from the controller. The gainvalue is dependent on the value of the yaw rate error and also on thecharacteristics of the surface on which the vehicle is running, ie. highMu or low Mu.

An example of a controller gain map which can be used is shown in FIG.2. For low yaw rate error, the scaling is negative, therefore reducingthe assist torque and making the steering feel slightly heavier. The aimis to produce a torque that increases with handwheel angle. For high yawrate errors, the gain is much higher, giving a large positive outputthat greatly increases the assist torque and makes the handwheel feellight. The shape of these maps can be varied to produce the desired feelin the steering system.

A single map can be used or, preferably, a plurality of maps can beavailable, the most suitable of which to suit the prevailingcircumstances can be selected automatically from a judgement of roadsurface conditions based, for example, on the measured yaw moment errorand column torque, or on tyre slip or video detection routines.

Improvement in the haptic information that the driver receives isprovided in the above described system by altering the torque in thesteering column in two ways. In the broadly linear operating region atconstant forward speed and before terminal understeer is reached, thetorque in the steering column is gradually increased as the handwheelangle is increased. This provides the driver with a haptic indicationvia the handwheel of the amount of lateral acceleration on the vehicle.When terminal understeer is reached such that additional handwheel anglefails to increase the vehicle yaw rate, the torque in the steeringcolumn is greatly reduced. This causes the handwheel to become verylight, providing the driver with an indication that the limit oftraction has been reached. Turning of the controller allows this drop intorque to happen slightly ahead of the actual loss of traction,providing the driver with a short but usable response time before steerauthority is lost.

Traditional power steering systems attempt to control the torque appliedby the driver to within limits. This can easily lead to a system wherethere are none of the haptic features described above and may havelittle or no change in torque with vehicle dynamic state. The design ofthe steering geometry also has a significant effect on the feel of thesteering. If the geometry is such that there is no build up of steeringtorque, or drop in torque once the limit is reached, then no simplepower steering system will be able to put that feel back. The presentsystem considers what yaw rate the driver is demanding and the actualyaw rate of the vehicle to determine what the torque in the steeringsystem should be. The assist torque generated by the power steeringsystem is then adjusted accordingly. The proposed system thereforeproduces steering feel which is independent of the power steering systemand the steering geometry.

Referring now to FIG. 3 there is shown a further development of thefirst aspect of the invention illustrated in FIG. 1, essentially being aderivative function of the steady state controller of FIG. 1 andproviding dynamic understeer haptic torque based on yaw rate estimation.In the controller of FIG. 3, use is again made of steer angle andlongitudinal velocity as in the first aspect but there is furtherincluded a dynamic component, in this case yaw rate. The resultingoutput provides a yaw rate error which when corrected or scaled againstdriver torque provides a haptic torque, which is optionally limited, foraddition to the power assist torque generated by the power assistsystem.

Referring next to FIG. 4 there is shown an alternative development,similar to FIG. 3, but which substitutes lateral acceleration for yawrate, thereby providing dynamic understeer haptic torque based onlateral acceleration estimation. In the controller of FIG. 4, use isagain made of steer angle and longitudinal velocity but there isincluded the further dynamic component of lateral acceleration insteadof yaw rate. The resulting output provides a lateral acceleration errorwhich when corrected or sealed against driver torque provides a haptictorque, which is optionally limited, for addition to the power assisttorque generated by the power assist system.

FIG. 5 illustrates the provision of a modifying torque based uponlateral acceleration to give a haptic response as lateral acceleration(and hence cornering force) increases up to the point of impendingundersteer.

FIG. 6 illustrates an arrangement for providing haptic torque based onany of the arrangements of FIGS. 1, 3 or 4 and including lateralacceleration feedback as shown in FIG. 5. This illustrates the overallsystem where a haptic torque based upon lateral acceleration can be usedto provide a dynamic response up to the point of impending understeerand where a dynamic function, such as yaw rate or lateral accelerationcan be further added to provide an additional response at the point ofimpending understeer.

In the embodiment of FIG. 7, the dynamic function is yaw rate and thedynamic error is therefore yaw rate error. The yaw rate estimator 30uses vehicle state information to calculate the yaw rate that the driveris demanding based on the assumption that the vehicle is linear. This isthen passed to the understeer control 32, which calculates a correctionto the assist torque being generated by the steering system. Inaddition, the lateral acceleration block 34 uses a lateral accelerationsignal to calculate a separate torque correction signal. The differencebetween these two correction signals at 36 becomes the Haptic (feel)torque which is added to the steering system assist torque at 38.

Yaw Rate Estimator and Reverse Detection

There are a number of ways in which the yaw rate or lateral accelerationof the vehicle can be estimated. Steady state estimation is thesimplest, but has been found to be unreliable at low vehicle speeds.Dynamic estimation is more complicated, but to date, no insurmountableproblems have been encountered. The final solution is to use informationfrom within the VSC. One further possible solution is to use lateralacceleration instead of yaw rate, as described in the last paragraph. Atthe same time as the yaw rate or lateral acceleration is beingestimated, a check is made to detect whether the vehicle is reversing.If the vehicle is reversing, then the yaw rate error or lateralacceleration error is set to zero. This prevents the understeercontroller affecting the steering feel while the vehicle is reversingwhich may be confusing for the driver. Yaw rate estimation and reversedetection is illustrated in FIG. 8.

Yaw Rate Estimator—Steady State Estimation

As described hereinbefore, yaw rate estimation can be based on thesteady state understeer equation;

$\hat{r} = {\frac{V_{x}}{l\mspace{11mu}\left( {1 + \left( {V_{x}/V_{c\; h}} \right)^{2}} \right)}\frac{\delta_{sw}}{G_{s}}}$where V_(X) is the vehicle longitudinal velocity, l is the wheel base,V_(ch) is the vehicle characteristic speed, δ_(SW) is the handwheelangle and G_(S) is the gain of the steering system from road wheels tohandwheel. This estimated value is then passed through a first order lowpass filter, tuned to give the estimate similar lag to the vehicle.Careful selection of the break point in this filter allows the pointwhere the steering goes light in relation to the loss of steer authorityto be controlled.

Experience with this method has highlighted a number of undesirablefeatures. The estimator breaks down at low speed causing the controllerto activate in “parking” type manoeuvres where it is highly undesirable.Also the performance of the estimator does not reflect changes in thevehicle performance as the speed varies.

Dynamic Yaw Rate Estimator

The yaw rate estimator can be improved by using a full order observerbased on the bicycle model with yaw rate feedback, see FIG. 9. Thebicycle model is written as:

$\begin{matrix}\begin{matrix}{{\frac{\mathbb{d}\;}{\mathbb{d}t}\begin{bmatrix}V_{y} \\r\end{bmatrix}} = \underset{\underset{A}{︸}}{\begin{bmatrix}\frac{- \left( {C_{af} + C_{a\; r}} \right)}{m\; V} & {\frac{- \left( {{a\; C_{af}} - {b\; C_{ar}}} \right)}{m\; V} - V} \\\frac{- \left( {{a\; C_{af}} - {b\; C_{ar}}} \right)}{I_{zz}\; V} & \frac{- \left( {{a^{2}\; C_{af}} + {b^{2}C_{ar}}} \right)}{I_{zz}\; V}\end{bmatrix}}} \\{\begin{bmatrix}V_{y} \\r\end{bmatrix} + {\underset{\underset{B}{︸}}{\begin{bmatrix}\frac{C_{af}}{m} \\\frac{C_{ar}}{I_{zz}}\end{bmatrix}}{\delta(t)}}}\end{matrix} & (1) \\{r_{meas} = {\underset{\underset{C}{︸}}{\begin{bmatrix}0 & 1\end{bmatrix}}\begin{bmatrix}V_{y} \\r\end{bmatrix}}} & (2)\end{matrix}$

The closed loop form of the estimator is written as:

$\begin{matrix}{{\frac{\mathbb{d}\;}{\mathbb{d}t}\begin{bmatrix}{\hat{V}}_{y} \\\hat{r}\end{bmatrix}} = {{\left\lbrack {A - {GC}} \right\rbrack\begin{bmatrix}{\hat{V}}_{y} \\\hat{r}\end{bmatrix}} + {B\;{\delta(t)}} + {G\; r_{meas}}}} & (3)\end{matrix}$with G selected to place the poles of the estimator at locations 10times faster than the open loop model. At first this seems unnecessaryas the actual yaw rate of the vehicle is being used in an estimator todetermine the yaw rate of the vehicle. It is true that at low yaw rates,the estimated and measured values will coincide. However, the estimatoris based upon an entirely linear model, which becomes less accurate asthe vehicle becomes more non-linear during increasing understeer. Hencewhen the vehicle starts to understeer, the estimated and actual yawrates start to diverge causing an error that is used to change theassist torque value. The use of yaw rate feedback therefore reduces thesusceptibility of the estimator to changes in dynamics with speed.

Experimentation has also shown that the estimator is robust to parameterchange though this can be further improved by the use of lateralacceleration as an additional feedback signal.

Lateral Acceleration Estimation

FIG. 10 is a diagram corresponding to FIG. 9 but illustrating the casewhere the estimator is a lateral acceleration estimator block Thecontroller is substantially the same as for the yaw rate estimator butwith the addition of the term D′ from the lateral acceleration equationbelow.

The system is the same as for the dynamic estimator, only the inputs tothe observer are steer angle and lateral acceleration (instead of yawrate), and the output is an estimate of lateral acceleration, Equation4. By subtracting the estimated lateral acceleration from the actuallateral acceleration, an error signal is generated that can be used inthe same way as the yaw rate error signal of the first embodiment ofFIG. 1

$\begin{matrix}{a_{y{({meas})}} = {{\underset{\underset{C^{\prime}}{︸}}{\begin{bmatrix}\frac{- \left( {C_{af} + C_{a\; r}} \right)}{m\; V} & \frac{- \left( {{a\; C_{af}} - {b\; C_{ar}}} \right)}{m\; V}\end{bmatrix}}\begin{bmatrix}V_{y} \\r\end{bmatrix}}\underset{\underset{D^{\prime}}{︸}}{\left\lbrack \frac{C_{af}}{m} \right\rbrack}{\delta(t)}}} & (4)\end{matrix}$The state equation for the lateral acceleration observer is given by:

$\begin{matrix}{{\frac{\mathbb{d}\;}{\mathbb{d}t}\begin{bmatrix}{\hat{V}}_{y} \\\hat{r}\end{bmatrix}} = {{{\left\lbrack {A - {G^{\prime}C^{\prime}}} \right\rbrack\begin{bmatrix}{\hat{V}}_{y} \\\hat{r}\end{bmatrix}}\mspace{11mu}\overset{\Cap}{x}} + {\left\lbrack {B - \;{G^{\prime}D^{\prime}}} \right\rbrack{\delta(t)}} + {{G\;}^{\prime}a_{y{({meas})}}}}} & (5)\end{matrix}$Reverse Detection

A preferred reverse detection algorithm is depicted in FIG. 11. Thereverse flag is zero, indicating that the vehicle is reversing, wheneither yaw rate is in the deadzone, or they have different signs. Thedeadzones 40, 42 limit the effect of noise on the yaw rate signals. Theyare set such that for a stationary vehicle, the noise on the yaw ratedoes not exceed the deadzone. While the signal is in the deadzone, thenthe output of the block is zero. The sign blocks 44, 46 take the sign ofthe input signal. If the input signal is zero, then the sign value isalso zero, otherwise it is plus if the input is positive or minus one ifthe input signal is negative. The saturation 48 is set so that if theinput is 1 or greater, then the output is 1. If the input is zero orless, then the output is zero. This limits the reverse flag to 0 or 1.Reverse detection could equally well be achieved by comparison ofestimated and actual lateral acceleration.

The yaw rate error, i.e., the difference between the yaw rate that thedriver is demanding and the actual yaw rate of the vehicle, is found bysubtracting the actual vehicle yaw rate from the estimated yaw rate asindicated in FIG. 8 at 50.

The Driver Torque Limit block limits the value of driver torque topre-defined levels. As the amount of torque that the understeer controlinjects into the system is proportional to the driver torque, limits onthe driver torque effectively place limits on the amount of torque thatthe system can inject.

Understeer Control

The understeer control comprises a number of sections as indicated inFIG. 12. The yaw rate error to scaling map 52 controls the steeringfeel, while the torque control 54 prevents excessive understeer torque,cancelling the assistance torque from the power steering, and evenreversing the torque applied by the driver. Again, the control structureof FIG. 12 could equally well produce an understeer torque based upon alateral acceleration error estimate.

Yaw Rate Error to Scaling Map

The input yaw rate error is used as the index input into a look-uptable. The output of this look-up is the driver torque scaling. Theelements of the look-up table are low for small yaw rate errors andlarge for high yaw rate errors as indicated in FIG. 13 (Yaw Rate Errorto Driver Torque Scaling Look-up Table). Again, the principles of FIG.13 could be applied equally to lateral acceleration error.

By varying the elements in the look-up table, the effect of thecontroller can be tuned to different vehicles and to give differentperformances. By making the transition from high to low occur at loweryaw rate errors, the steering feel can be made to go light well beforethe car reaches terminal understeer. If the torque scaling gain is madeslightly negative initially, then the steering can be made to feelheavier at low yaw rates. This is very similar to an increase in drivertorque in proportion to lateral acceleration.

Torque Control

It is possible for the understeer controller to generate a torque thatis larger than the assist torque being generated by the power steeringsystem. If this occurred, the torque felt by the driver would bereversed. Not only would this be uncomfortable and confusing, therewould be the potential that the steering would wind on more lock insteadof returning to straight ahead when the driver removed his hands. Toprevent this torque reversal, the algorithm shown in FIG. 14 (blockdiagram of torque control section) has been constructed. This attemptsto keep the driver torque above a predefined torque threshold which isachieved by comparing at 56 the actual driver torque with the torquethreshold and generating a scaling between 0 and 1. By applying thisscaling to the output of the understeer controller, the effects of theundersteer controller can be controlled.

The algorithm is designed such that a scaling correction value isproduced with the shape shown in FIG. 15 where T_(threshold) is thetorque value below which the driver torque should not fall and k_(tune)is the value of tuning gain.

The scaling correction has 3 stages. In the first stage, the drivertorque is very low and it is essential to quickly limit the effect ofthe understeer controller to prevent torque reversal. This is achievedby setting the scaling correction to zero, thereby nullifying the effectof the understeer controller. This is in effect a deadzone and is usefulin dealing with noise from the torque sensor. The second stage isbetween some designed point and the torque threshold value. In thissection, the output from the torque controller ramps linearly from 0 to1 producing smooth control of the understeer controller output. The lastsection is where the driver torque is above the torque threshold. Thescaling correction is one, therefore having no effect on the understeercontroller output.

Torque Threshold

This block 58 is a constant equal to the level at which is it desiredthat the driver torque does not fall below. A satisfactory value hatbeen found to be 1 Nm.

Max

The MAX blocks 60, 62 simply take the maximum value of their two inputs.In both cases they prevent the input failing below zero which couldcause a negative scaling correction. This would provide a torquereversal at the driver, which would be highly undesirable as it wouldconfusing for the driver.

Tuning Gain & Filter

The tuning gain and filter block 64 smooths the driver torque signal sothat smoother control with less switching is produced. The timing gainis used to control the size of the deadband as described by:

${T_{deadband} = \frac{{T_{threshold}k_{tune}} - 1}{k_{tune}}}{\mspace{25mu}\;}$Lateral Acceleration Control

The lateral acceleration feel control, FIG. 16, is designed to produce abuild up of torque in the steering system in proportion to the lateralacceleration of the vehicle.

Lateral Acceleration Compensation (66)

The lateral acceleration value supplied by a lateral acceleration sensoris passed through a compensation algorithm. This may simply be a filterto remove noise, or it may be a correction for the lateral accelerationsensor position. The best results are obtained if the lateralacceleration at the front axle is used. As the sensor is unlikely to beplaced there, then the compensation element can be used to compensatefor physical displacement of the sensor from a position mid way betweenthe front wheels where the measured lateral acceleration wouldpreferably be measured.

ABS (68)

The ABS block takes only the magnitude, thereby ensuring that thescaling is always positive and the lateral acceleration torque alwayshas the same sign as the driver torque. If the relative signs were tochange, then the steering feel would be confusing.

Look-up Table (70)

The gain that is applied to that lateral acceleration is dependent onspeed and is determined in the look-up table. At low speeds, the gain iszero, between around 10 and 40 kph the gain rises to a peak value andthen remains at there at all higher speeds. A typical peak gain value isaround 0.3.

Haptic Torque Limit

This is a saturation that prevents the haptic torque exceedingpredefined limits. If it does exceed these limits, then it is simplyheld at the limit.

The major features of a preferred form of the Haptic Controller can besummarised as follows.

Torque Change in Understeer can be Artificially Introduced

If the steering system has been designed such that there is insufficientdriver torque drop off when the vehicle is in understeer, then this canbe added using information from other sensors on the vehicle.

Torque Build up in Proportion to Lateral Acceleration can beArtificially Introduced

If the steering system has been designed such that there is insufficientdriver torque build up in proportion to lateral acceleration, then thiscan be added using information from other sensors on the vehicle.

Applied Torque is Proportional to Driver Torque

By making the torque applied by the controllers proportional to drivertorque, passive vehicle performance is maintained. If the driver removestheir hands from the handwheel, the driver torque falls to zero, thehaptic controller torque also falls to zero and the handwheelself-centres.

Point of Application of Understeer Control is Entirely Tuneable

The point at which the steering becomes light can be isolated from thevehicle. Therefor it is possible to provide for the steering becominglight slightly before terminal understeer, thereby providing the driverwith advanced warning of a loss of traction.

Amount of Torque Drop off in Understeer is Tuneable

The amount that the torque drops off in understeer is tuneable insoftware. Therefore it is simple to create a steering system that has atorque drop off characteristic that is suited to the vehicle.

Amount of Torque Build up in Lateral Acceleration is Tunable

The amount that the torque builds up in proportion to lateralacceleration is tuneable in software. Therefore it is simple to create asteering system that has a torque drop off characteristic that is suitedto the vehicle, including mapping it with speed to give a suitable lowspeed characteristic.

Lateral Acceleration can be Used Instead of Yaw Rate

The yaw rate signal can be substituted for a lateral accelerationsignal, and the system will work in the same way with only a few changesto the estimator equations. This is advantageous as the cost of alateral acceleration sensor is much less than a yaw rate sensor.

Algorithm Does not Affect Steering Feel When Reversing

A simple algorithm detects when the vehicle is reversing based on therelative signs of the yaw rate and the yaw rate estimate. By preventingthe controller acting when the vehicle is reversing, confusing changesin steering feel are avoided.

Minimal Additional Hardware

There are minimal additional hardware requirements beyond the basevehicle. One or more cheap lateral acceleration sensors may be all thatis required

Glossary of symbols Parameter Parameters l Wheel base V_(ch) Vehiclecharacteristic speed G_(s) Steering ratio C_(af) Front tyre lateralstiffness (per axle) C_(ar) Rear tyre lateral stiffness (per axle) mVehicle mass l_(zz) Vehicle yaw moment of inertia a Longitudinaldistance from vehicle cog to front axle b Longitudinal distance fromvehicle cog to rear axle A Bicycle model state transition matrix (2 × 2matrix) B Bicycle model state input matrix (2 × 1 matrix) C Bicyclemodel state output matrix G Observer gain matrix (2 × 1 matrix) D′Alternative bicycle model output feedforward matrix (using lateralacceleration as a model output) C′ Alternative bicycle model stateoutput matrix (using lateral acceleration as a model output) G′Alternative observer gain matrix (using lateral acceleration as a modeloutput). Signals {circumflex over (r)} Yaw rate estimate V_(x), VVehicle longitudinal velocity δ_(sw) Steer angle (handwheel) V_(y)Vehicle lateral velocity r Vehicle yaw rate t Time δ(t) Steer angle(road wheel) r_(meas) Measured yaw rate {circumflex over (V)}_(y)Estimated lateral velocity {circumflex over (r)} Estimated vehicle yawrate a_(y(meas)) Measured lateral acceleration

In accordance with the provisions of the patent statutes, the principleand mode of operation of this invention have been explained andillustrated in its preferred embodiment. However, it must be understoodthat this invention may be practiced otherwise than as specificallyexplained and illustrated without departing from its spirit or scope.

1. A controller for a power assisted steering system for a motor drivenroad vehicle, the controller comprising: an assist torque signalgenerating means arranged to generate an assist torque signal for thevehicle power steering system in response to the driver's applied torqueand sensed vehicle speed and effective to reduce the driver's steeringeffort; and a means for generating a haptic torque signal based upon avehicle dynamic error which is arranged to be added to the torque assistsignal such that when said dynamic error builds up, corresponding toincreasing steering instability of the vehicle, the haptic torque signaladded to the torque assist signal reduces the effective road reactionfeedback sensed by the driver in advance of any actual vehicle stabilityloss whereby to allow the driver to correct appropriately in good timebefore terminal steering instability is reached.
 2. The controlleraccording to claim 1, wherein said vehicle dynamic error is a vehicleyaw rate error and said means for generating a haptic torque signalincludes a yaw rate estimator and further wherein said haptic torquesignal is based upon said vehicle yaw rate error which is arranged to beadded to the torque assist signal such that when the yaw rate errorbuilds up, corresponding to increasing steering instability of thevehicle, the haptic torque signal added to the torque assist signalreduces the effective road reaction feedback sensed by the driver inadvance of any actual vehicle stability loss whereby to allow the driverto correct appropriately in good time before terminal steeringinstability is reached.
 3. The controller according to claim 1, whereinsaid said vehicle dynamic error is a vehicle lateral acceleration errorand means for generating a haptic torque signal includes a lateralacceleration estimator and further wherein said haptic torque signal isbased upon said vehicle lateral acceleration error which is arranged tobe added to the torque assist signal such that when the lateralacceleration error builds up, corresponding to increasing steeringinstability of the vehicle, the haptic torque signal added to the torqueassist signal reduces the effective road reaction feedback sensed by thedriver in advance of any actual vehicle stability loss whereby to allowthe driver to correct appropriately in good time before terminalsteering instability is reached.
 4. The controller as claimed in claim2, wherein the assist torque signal generating means includes anelectric motor.
 5. The controller as claimed in claim 2, wherein the yawrate error is established by comparing an estimated yaw rate derivedfrom measured values of steering angle and vehicle longitudinalvelocity, with measured vehicle yaw rate.
 6. The controller as claimedin claim 5, wherein the yaw rate error is saturated, if necessary, toprevent excessive demand and scaled by a gain map.
 7. The controller asclaimed in claim 6, wherein the gain is controlled in accordance withyaw rate error, a low yaw rate resulting in a relatively low gain and ahigh yaw rate resulting in a relatively large gain so as to increase theassist torque from the power steering and make the steering feel lightto the driver.
 8. The controller as claimed in claim 6, wherein aplurality of gain maps are provided, the most suitable to comply withthe prevailing conditions being arranged to be selected automaticallyfrom a judgement of road surface conditions based on measured data. 9.The controller as claimed in claim 6, wherein the haptic torque signalis established by scaling the steering column torque using the scaledyaw rate error, the haptic torque signal being added to the torqueassist signal to provide an output for driving the electric motor. 10.The controller as claimed in claim 2, wherein the controller furtherincludes a reverse detection device for detecting whether the vehicle isreversing and comprising means for detecting when the actual yaw ratelies in a deadzone.
 11. The controller as claimed in claim 10, whereinthe controller further includes a reverse detection device for detectingwhether the vehicle is reversing and comprising means for detecting whenthe estimated yaw rate lies in a deadzone.
 12. The controller as claimedin claim 11, wherein the controller further includes a reverse detectiondevice for detecting whether the vehicle is reversing and comprisingmeans for detecting when the actual yaw rate and the estimated yaw ratehave different signs.
 13. The controller as claimed in claim 3, whereinthe controller further includes a reverse detection device for detectingwhether the vehicle is reversing and comprising means for detecting whenthe actual lateral acceleration lies in a deadzone.
 14. The controlleras claimed in claim 13, wherein the controller further includes areverse detection device for detecting whether the vehicle is reversingand comprising means for detecting when the estimated lateralacceleration lies in a deadzone.
 15. The controller as claimed in claim14, wherein the controller further includes a reverse detection devicefor detecting whether the vehicle is reversing and comprising means fordetecting when the actual lateral acceleration and the estimated lateralacceleration have different signs.
 16. The controller as claimed inclaim 3, wherein the assist torque signal generating means includes anelectric motor.
 17. The controller as claimed in claim 1, wherein thehaptic torque signal generating means includes a dynamic estimatoradapted to produce said dynamic error based on measured values of steerangle, vehicle dynamic rate and vehicle longitudinal velocity.
 18. Thecontroller as claimed in claim 17 wherein the controller furtherincludes an understeer control which produces an understeer torque fromsaid dynamic error and a limited value of driver applied torque.
 19. Thecontroller as claimed in claim 18, wherein the understeer torqueprovides said haptic torque signal which is added to said assist torquesignal to drive an electric motor of the power assisted steering system.20. The controller as claimed in claim 19, wherein the understeer torqueis modified by a feedback signal corresponding to a lateral accelerationtorque that is based on a measured lateral acceleration of the vehicleand the vehicle longitudinal velocity.
 21. The controller as claimed inclaim 20, in which the lateral acceleration torque is further dependenton the driver applied torque on the steering wheel.
 22. The controlleras claimed in claim 20, wherein the lateral acceleration torque isdeveloped in dependence upon the product of functions dependent uponlateral acceleration and longitudinal velocity.
 23. The controller asclaimed in claim 18, wherein the understeer control includes a torquecontrol providing a scaling correction gain for correcting the dynamicerror signal in dependence upon driver applied torque.
 24. Thecontroller as claimed in claim 23, wherein the torque control is adaptedto seek to keep the driver torque above a predefined torque threshold bycomparing the actual driver torque with a torque threshold andgenerating a scaling between 0 and 1, the scaling being applied to theoutput of the understeer control.